The present invention relates to the measurement of the current density distribution in electron and ion beams, particularly to a technique using a modified Faraday cup having a narrow slit to create an image of the current density of such beams, and more particularly to a simplified technique using a modified Faraday cup having a disk with radial slits and a single computer to acquire the data and create an image of the current density of such beams.
A system and process for tomographic determination of the power distribution in electron beams, as described and claimed in above-referenced copending application Ser. No. 08/283,438 and U.S. Pat. No. 5,382,895, allowed a user to measure the power distribution and thus be able to tailor it to the user's specific needs. That system utilized a modified Faraday cup to acquire profiles of the electron beam at a number of different angles. The modified Faraday cup includes a slit that is much narrower than the width of the beam in front of the Faraday cup that collects the portion of the beam that passes through the slit when the beam is swept across the slit using deflection coils. This portion of the beam forms a current that travels to ground through a signal wire and a sensing resistor. The current creates a voltage across the resistor that is read by an analog to digital acquisition device. The data acquisition device thus saves a time record which contains a profile of the beam as it travels across the slit. Beam profiles are taken at a number of different angles by rotating the slit and sweeping the beam across it. The system uses these profiles to do a reconstruction of the beam power density distribution using mathematical techniques originally developed for the non-destructive evaluation of solid objects using x-rays.
A disadvantage of the above-referenced system and process is that acquiring data requires a repetitive sequence of rotating the slit into position, turning on the beam and sweeping it, recording the beam profile, and stopping the beam before moving the slit to the next angular position. A full set of beam profile data can take as much as ten minutes to acquire and the results may be adversely affected by an unstable beam, due to the beam being repetitively turned on and off.
The present invention overcomes the above-described disadvantage of the system and process of the above-referenced copending application and patent, by eliminating this repetitive beam profile data acquisition process by utilizing a disk with a number of radial slits, eliminating the need for a stepper motor for rotating the slit and its associated wiring, and allows the necessary data to be taken in a fraction of a second, thus improving the quality of the data. In addition, a single computer is used to generate the signals actuating the sweep of the beam around the radially slit disk, to acquire the data, and to do the reconstruction. Thus, the present invention significantly reduces the apparatus and the time for determining the power distribution in electron beams.